Method and apparatus for electroperforating sheet material



April 24, 1951 J. w. MEAKER ET AL METHOD AND APPARATUS FORELECTROPERFORATING SHEET MATERIAL 2 Sheets$heet 1 Filed Oct. 25, 1946INVENTORJ Jaw/v H WA/QVP J W MEAKER ETAL METHOD AND APPARATUS FORELECTROPERFORATING SHEET MATERIAL April 24, 1951 Filed Oct. 25, 1946Patented Apr. 24, 1951 2,550,366 METHOD AND APPARATUS FOR ELECTRO-PERFO'RATING SHEET John W. Meakcr, New York,

H. McShan,

signor to said Meaker Application October 25, 1946, Serial No.

2 Claims. 1

Thi invention relates to a method and an apparatus for perforating sheetmaterial by an electric discharge and more particularly to a method andan apparatus for the electro-perforation of sheet material havingrelatively high dielectric properties.

An object of this invention is to provide a method and an apparatus tofacilitate the electroperforation of sheet materials having relativelyhigh dielectric strength such as plastics, Koroseal, leather,leatherette, cellulose products, rubberized or plastic coated fabricsand the like. The invention is particularly adapted for the perforationof relatively thin sheets of such materials as ordinarily used forprotective coverings of various sorts. The process and apparatusdescribed and claimed herein is especially suitable forelectro-perforating thin sheets of such material without damaging thematerial mechanically or affecting its appearance.

Reference is made to Patent No. issued October 30, 1945, to John W.Meaker and Edward H. Yonkers, Jr., in which the difficulties experiencedin the electro-perforation of sheet material having high dielectricstrength, unless the material is pretreated to reduce its dielectricstrength are described in detail.

A preferred embodiment of the invention is illustrated in theaccompanying drawings wherein:

Fig. l is a perspective view of an apparatus suitable for carrying outthe invention having the electricai connections thereto illustrateddiagrammatically;

Fig. 2 is a perspective view in section of a portion of a sheet ofmaterial prior to treatment;

Fig. 3 is a similar view of the material after it has been abraded;

Fig. 4 is a similar view of the material after it has been abraded andthen perforated by an electric discharge;

Fig. 5 is a perspective view illustrating a modified apparatus forelectrically perforating the ma terial; and

Fig. 6 is a side View abrading the material.

Referring to the drawings in detail, a thin sheet of material It, whichis to be treated, is fed from a roll I l and passes between a pair ofpressure rollers l2 and I3. The pressure rollelS l2 and 13 arepreferably hard surfaced rolls made of steel or the like, and the upperroll I2 has a sheet of abrasive carrying material l4 fixed to itssurface. However, the abrasive carrying material may, if desired, beplaced on both of the T0115.

of a modified apparatus for MATERIAL N. Y., and Clarence Newark, N. J.,said McShan as- The lower roll I3 is journalled at each end in supportsl5. The upper roll I2 is journalled at each end in bearing blocks 16which are slidably mounted in the supports l5. The pressure exertedbetween the rolls may beadjusted by pressure adjusting screws ll whichextend through the upper portion of the support 15 and bear against thebearing blocks 16. If desired, compression springs may be installedbetween the rolls and the adjusting screws. It has been found that apressure of from 30 to lbs. between the rolls will produce satisfactoryindentation of the surface of a sheet of Koroseal six thousandths of aninch .006") thick for subsequent electro-perforation. However, it may befound terial to be indented and the thickness thereof. Thi treatmentproduces a series 0f indentations Illa in the sheet 10 which varysomewhat in depth and which are distributed at random over the surfaceof the material as illustrated in Fig. 3. The indentations have a lowerdielectric strength than the untreated fabric with the deepest abrasionshaving the lowest dielectric strength. Thus, materials when indented inthis manner may be more readily penetrated by an electric discharge.

After the sheet material has been indented, it is passed between a setof spaced electrodes I8 and IS. The spaced electrodes l8 and I!) arepositioned. relative to the pressure rolls I2 and i3 and extend acrossthe width of the sheet material to be perforated. The upper electrode 18may consist of a series of individual electrodes connected electricallyto each other and pivotally carried by a support 25. The upper electrodemay extend across the entire width of the material or a number of rowsof such electrodes may be provided to cover the entire width of thematerial by placing the electrodes in the various rows in staggeredrelation to each other. The lower electrode [9 may be a rod or series ofrods positioned beneath the upper electrodes.

The electrodes I 8 and 19 are connected to opposite sides of thesecondary of a transformer the primary of which is connected to a sourceof electric energy S. Thus, the transformer T supplies to the electrodesa high tension, alternating current having such characteristics that anelectric discharge will pas between the electrodes and through theindented material as it passes between the electrodes therebyelectroperforating the material. After the sheet has been perforated, itis taken up in a roll 2| that may be driven by any suitable means.

As previously mentioned, the discharge between the electrodes will occurat the point in the material where the resistance to the passage of suchdischarge is lowest due to the indentation. Since the indentations aredistributed at random over the surface of the material, as illustratedin Fig. 4, the perforations illb are also distributed at random over thesurface of the material so that there is no definite pattern orarrangement of the perforations.

The abrasive carrying material [4 may be sand-paper, garnet paper, emerycloth or the like, and may be either fine, coarse, or medium grade. Thegrade of the abrasive material will depend upon the resistance of thematerial being treated to such indentation, the thickness of thematerial and the size of the perforation or porosity desired. Normally,a fine grade of abrasive material may be used on thin material such asthe Korcseal sheet mentioned. The main requirement in this respect is tohave an abrasive material with a grain of sufiicient size to penetrateor indent the material substantially, and thicker materials normallyrequire an abrasive material with a larger size grain. It is alsodesirable that the recesses between the grains of the abrasive materialbe approximately equal to or greater than the thickness of the materialbeing treated.

As a coarse grade of abrasive material will produce a smaller number ofindentations than a finer grade for any given area, there will be fewerperforations per square inch produced in the subsequent treatment of thematerial as the grain size of the abrasive material increases. However,an adequate number of perforations will be obtained even under suchconditions as the number of indentations on the material will normallyexceed the number of the electric discharges which are passed throughany given area. Where there is an excess of indentations, the electricdischarge will pass through the deepest of the indentations, or the onehaving the lowest dielectric strength of those between the electrodes ata given instant. For a given number of holes per square inch, thedielectric strength of the material should not exceed a maximum valuedetermined by the equation:

where K equals dielectric strength along the surface of the material(approximately that of air). D equals the distance between indentationsto be electro-perforated and an adjacent hole perforated. t equalsthickness of material, and k equals dielectric strength of anindentation. If tk is greater than K (2D+t) then the electric dischargewill occur through a previously perforated hole, rather than through anew indentation. Therefore, the value of It must be reduced sufficientlyby weakening the material as described to obtain a given number of holesper given area. This is accomplished by employing an abrasive ofsuihcient size to meet such requirements.

While emery cloth or the like has been specifically mentioned above,various types of abrasive materials may be used satisfactorily. Forexample, a solid Carborundum wheel may replace the steel roller coveredwith sandpaper or a steel roller having a roughened surface may be used.This apparatus and method may also be used in perforating materialswhich do not have particularly high dielectric strength. For example, inperforating cellophane for use as a container for material such as tea,from which an infusion is to be made, it has been found particularlyadvantageous to emboss the cellophane sheet by means of a fine meshmetal screen carried on one of the rollers. This indents the cellophanesheet as it passes between the rollers and embosses it with a regularpattern. When the cellophane is then subjected to an electric discharge,the discharge will pass through the indentations eliminating thetendency of the holes to run together and thereby weaken the materialwhen such indentations are not present.

The porosity or the number of perforations per square inch, may also becontrolled by the speed at which the material travels and the frequency,voltage and current of the electric discharge, all of which will begoverned by the characteristics of the material being treated and theresults which it is desired to obtain. Inasmuch as the regulation ofthese factors is well understood by those skilled in the art, they willnot be described in detail here.

By this means, the size and distribution of the perforations may becontrolled so that perforations may be obtained that will permit thepassage of air through the material for ventilation or other purposes,but which will not materially decrease the materials resistance to waterpenetration or destroy the water repellant characteristics of thematerial, if any. Also, in the case of the cellophane tea bag, largerperforations may be obtained, for instance, by increasing the current,that will readily permit water penetration if that is desired.

With material of extremely high dielectric strength, it may be founddesirable to moisten the surface of the material just prior to itselectro-perforation in order to reduce the dielectric strength of thematerial. The moistening of the material assists the passage of theelectric discharge through the material, and it also has a tendency toprevent carbonization which appears as unsightly black rings at theperforations in some materials.

In the modified form of electric discharge apparatus illustrated in Fig.5, a piece of copper screening 22 has been substituted for the pivotedelectrodes l8. This copper screen 22 extends across the width of thematerial to be treatedand is carried by a support 23. The screenelectrode 22 extends downwardly at an angle and contacts the surface ofthe material to be treated above the lower electrode 19. Thisarrangement provides an electrode which is flexible and which is ofsimple construction. A flexible metal plate may also be used as anelectrode with very satisfactory results.

In the modified form of abrading device shown in Fig. 6, there are twosets of oppositely disposed pressure rolls 24, and an endless belt 25 ofabrasive carrying material is placed around each set of the pressurerolls. The sheet material l0 passes between the opposed surfaces of theabrading material and frictionally engages therewith so that abrasivematerial is moved by the sheet at the same speed as the sheet material.This arrangement provides an indenting surface which will not requirereplacement or renewal as frequently as the single rolls. If desired, asingle endless belt may be used on one set of the rolls to indent onlyone surface of the material.

While a preferred embodiment of the invention has been described andillustrated herein, it will be understood that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:

1. A method of electro-perfor-ating sheet material, comprising the stepsof applying an abrasive carrying material to a surface of said sheetmaterial under sufficient pressure to produce a series of indentationsdistributed at random over the surface of the material, and then passingsaid sheet material between a pair of spaced electrodes extendingsubstantially across the width of said indented surface and perforatingsaid material at random by discharging an electric current between saidelectrodes and through said material.

2. An apparatus for electi e-perforating sheet material of a relativelyhigh dielectric strength including a pair of oppositely disposed rolls,said rolls being positioned to engage with the opposite surfaces of amoving web of sheet material, an abrasive material carried by at leastone of the rolls and contacting under pressure with one sur- REFERENCESCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 692,834 Davis Feb. 11, 19021,410,899 Duncan Mar. 28, 1922 2,141,869 Kom'g Dec. 27, 1938 2,388,069Meaker et a1 Oct. 30, 1945

